Make scan paths required operands; add -x/--one-file-system

scan now takes one or more PATH operands (directories or regular
files) via cobra flags instead of the -root flag with its /srv
default; invoking scan with no operand is a usage error and a
nonexistent operand is fatal. Filesystem boundaries are crossed by
default; the new -x/--one-file-system flag (GNU du/rsync convention)
stops the walk at each operand's filesystem, implemented by comparing
lstat device IDs with build-tagged helpers for darwin's int32 Dev.
Verified against a real mounted disk image: default crosses, -x does
not, --one-file-system is identical to -x.

main.go

@@ -6,7 +6,7 @@
 //
 // Usage:
 //
-//	sfdupes scan [-root /srv] [-workers N] > files.dat
+//	sfdupes scan [--workers N] [-x] PATH... > files.dat
 //	sfdupes report [files.dat|-] > dupes.tsv
 //	sfdupes trees [files.dat|-] > dupetrees.tsv
 //
@@ -16,6 +16,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"runtime"
 
 	"github.com/spf13/cobra"
 )
@@ -52,16 +53,23 @@ func main() {
 	root.SetErr(os.Stderr)
 	root.CompletionOptions.DisableDefaultCmd = true
 
+	var (
+		scanWorkers int
+		scanOneFS   bool
+	)
+
 	scanCmd := &cobra.Command{
-		Use:   "scan [-root /srv] [-workers N]",
-		Short: "Walk a tree and emit one record per regular file on stdout",
-		// The README specifies single-dash flags (-root, -workers);
-		// parse them with the stdlib flag package inside runScan.
-		DisableFlagParsing: true,
+		Use:   "scan [--workers N] [-x] PATH...",
+		Short: "Walk trees and emit one record per regular file on stdout",
+		Args:  cobra.MinimumNArgs(1),
 		Run: func(_ *cobra.Command, args []string) {
-			runScan(args)
+			runScan(args, scanWorkers, scanOneFS)
 		},
 	}
+	scanCmd.Flags().IntVar(&scanWorkers, "workers", runtime.NumCPU(),
+		"concurrent workers for the stat and hash passes")
+	scanCmd.Flags().BoolVarP(&scanOneFS, "one-file-system", "x", false,
+		"do not cross filesystem boundaries")
 
 	reportCmd := &cobra.Command{
 		Use:   "report [files.dat|-]",